The invention relates to controlling the operation of a paper, board or tissue production line at various stages by means of fluids used, such as lubricating, heat transfer or hydraulic fluids, in the equipment. The invention can especially be used in paper and/or board machines and in the finishing units thereof.
In paper and/or cardboard manufacture, for example, so-called long nip or shoe nip solutions can be used at the various stages. They comprise a rotating belt and a roll that presses against it so that the belt travels a fairly long way on the surface of the roll. The belt is supported in the nip by means of a shaped surface, so-called shoe. A thin film of oil can be spread between the shoe and the belt, often through a special oil pocket. The oil film allows the belt to slide on the shoe as frictionlessly as possible. Through the film, by setting the shoe and the flow state, forces that support the belt in the normal direction can be controlled. These forces are supporting forces that receive the nip pressure, and they can be used to adjust the pressure distribution of the nip both in the machine direction (MD, the length of the nip) and in the cross machine direction (CD, profiling). The long nip can be used in calenders in particular, and also in pressure apparatuses.
In traditional long nip solutions, the action of the nip can be influenced by selecting a desired oil and by adjusting the volume of the supply flow. The properties of the oil are relatively constant, excluding the change in viscosity caused by a possible change in the temperature. The volume of the supply flow can also be controlled in the cross machine direction, if the shoes and/or the outlet flow are divided into corresponding functional blocks.
In many devices used in paper and/or board manufacture, for example, include heat transfer by means of a fluid is included. For example, heated rolls, such as the thermo rolls of calenders, polymer rolls of calenders provided with balancing or cooling solutions, and, generally, heat exchangers. Water, steam or oil is generally used as the heating medium.
The effectiveness and the action of heat transfer, in general largely depend on the properties of the medium used and on the character of the flow. Firstly, the transfer of great heat powers requires a large flow velocity. Furthermore, the boundary-layer flow of the medium and of the surface that delivers or receives heat should be relatively abrupt, so that heat would transfer effectively enough over the boundary layer. Sufficient flow velocities can be provided using small flow channels, in which the wall surface of the channel is also relatively large compared with the cross-sectional area of the flow channel.
In traditional heat transfer solutions, the flow of the medium can only be influenced to a limited extent. For flow control, the only means available have been the design of the flow channels and different displacement and choking block solutions. These means have been used to try to keep the flow velocity of the medium optimal in each situation, for instance to equalize the heat transfer of a thermo roll in the cross machine direction. Solutions that increase the turbulence of the channel flow are also known. These include, for example, a spiral or the like placed in the channel, or roughening of the channel walls.
Paper webs can be pressed using a heated roll both in pressure apparatuses and calenders. The purpose in the pressure apparatus is to heat the web so that the viscosity of water decreases and thus the exit of water from the web is easier. In impulse drying, the idea is to heat the web so that the water evaporates and the increased pressure drives the water out of the web. The calender is used to heat the line so that the fibres become soft, whereby they are easier to work up.
Both in the pressure apparatus and the calender profiling tubular variable crown rolls are used. The roll is loaded from the inside by using shoe elements. These rolls can be used, for example, as coated backing rolls or heated thermo rolls.
Both in the pressure apparatus and the calender generally, a way to heat the roll, which is based on the flow of the heating medium, is used. The jacket of the roll has axial channels, through which the heating medium is conducted. To balance temperature differences, the medium in some channels goes in the opposite direction than in the others. To balance the temperature differences, a so-called displacement block solution has also been used, in which blocks placed in certain locations in the flow channel are used to decrease the cross-sectional area of the flow, whereby the flow velocity is increased. In that case, also the heat transfer is enhanced. Centre drilling can also be used as a flow channel in the rolls. A heating medium that flows in the channels is also used to cool unheated polymer rolls and to equalize the temperature differences. The heating medium is directed to the channels in the roll jacket through the ends of the roll by using the channels in the end pieces. One problem in the heat transfer is the changes in shape caused by irregular temperature distribution.
Many devices used in paper and/or cardboard manufacture contain regulating units of mechanical movement, such as dampers, switches or brakes. To control linear movements, for example, dampers are used to stop the movement of masses or to dampen oscillation or vibration. In rotary transmission lines, switches and brakes are used to move the torque. These regulating units are often hydraulic.